At present times, the production from renewable energy sources being steadily growing, “conventional” power plants will increasingly be required to take on additional tasks such as to provide complementary electricity production to the grid they are connected to on short notice, particularly in the absence of large-scale energy storage systems, which are still far away from commercialization. Large fluctuations during the day require power generators to react quickly to maintain the balance between demand and production. Under these circumstances, the power plants have to supply power to the grid in a flexible way: for example, when the energy required by the grid is low they must be able to reduce the power supplied to the grid down to zero and when the grid requires power again they must be able to provide it very quickly (in some cases they must be able to provide tens of megawatts in seconds).
In the last ten years, the key area of focus of conventional power sources has been the switch from base load to intermediate load operation, and thus the need for fast load ramps, shorter low-load and start-up times, and grid stabilization. In addition, the demand for ancillary services such as provision of control reserves and frequency support, as well as tertiary control reserves and load-follow operation, has increased significantly. As a result, new operating requirements have emerged, such as two-shift operation, load-follow operation, island operation, black start capability, frequency support and very high start-up and operating reliability, in order to stabilize power grid dynamics and hence ensure secure and economic electricity supply.
As the requirements for load cycling are changing and the expansion of renewables is increasing, “conventional” power plants will have to accommodate to periods in which there is an over- or under-capacity of power. Depending on the country and power grid concerned, various dynamic capabilities are required. Combined-cycle plants (i.e. power plants comprising gas and steam turbines) allow faster load changes within a wider load range, which make these plants more flexible. Furthermore, when considering fast start-ups and efficiency, the combined cycle power plant stands high in comparison with other electricity production methods. Even more, combined-cycle plants offer a significantly higher rate of load change than other conventional power plants thanks to innovative and specifically developed systems.
If, in future, the renewable capacity that is currently planned becomes operational, previously base loaded power plants, such as combined-cycle power plants, will not merely have to be run down to part load, but will have to be completely shut down in many cases in order to avoid significant overcapacities. These combined-cycle power plants will then need to be started up from the shut-down condition as rapidly as possible to cover demand in the event of short term loss of renewable power. The only solution, in the absence of adequate storage systems, is the increased use of conventional plants in so-called “two-shift operation”, that is, start-up and shut-down on a daily basis (and sometimes several times per day) in order to compensate for fluctuations in load. Under these operating conditions, it is essential that start-ups are able to take place very rapidly and reliably, which is possible with combined cycle plants, due to the relative simplicity of their fuel and combustion systems.
As it was mentioned, start-up reliability is becoming an increasingly important issue and combined-cycle plants exhibit significant advantages over other conventional technologies in this respect, due to the fact that they have the lowest degree of complexity. Several start-up methods for combined-cycle power plants are known in the state of the art, as per EP 2423462 A2, EP 0605156 A2, CN 202230373 U, for example. Enhanced start-ups are known as per US 2005/0268594 A1, US 2009/0126338 A1 or WO 2012/131575 A1, for instance.
It is also known in the state of the art, for example as per EP 2 056 421, a method to connect a combined power plant, comprising a gas turbine and a steam turbine, to a grid.
In combined-cycle power plants, having two power generating units, gas turbine and steam turbine, during start-up, gas turbine starts as the first unit to run up by increasing the shaft speed to nominal speed. After synchronization with the electrical network, gas turbine starts to supply power to the grid. Steam turbine is the second unit which can start when the steam conditions (such as steam pressure and steam temperature provided by the steam generator system) have reached the appropriate conditions for steam turbine components (such as the shaft, the casing and the steam admission valves). The colder the steam turbine materials are, the lower the steam conditions should be (lower steam pressure and lower steam temperature, for instance) in order to prevent extreme impact on steam turbine component lifetime. This may be achieved by the operation of the gas turbine at lowest possible load corresponding to the lowest possible gas turbine exhaust temperature and exhaust mass flow. In order to achieve steam turbine start-up requirements, the steam generating system such as heat recovery steam generators must be equipped with de-superheating stations which are capable to control the required steam temperature. In addition, the steam turbine by-pass stations must be also designed corresponding to steam turbine pressure requirement for the start-up.
Traditional start-up procedures require start of the steam turbine at low to intermediate gas turbine operation loads which leads to an overdesign of de-superheating stations and steam turbine bypass station. Furthermore, due to unpredictability of start-up load profile, the exported power during start-up is not paid by the grid authorities. The grid operator has to integrate gas and steam turbine exported power into the electrical network and manipulate the total grid power accordingly. Also the start-up fuel consumption costs must be covered by the plant owner. Therefore, the plant owner prefers to start the steam turbine at the lowest possible gas turbine load to decrease start-up costs.
Therefore, it would be desirable that the steam turbine start-up (running up and heating up) to the condition ready for loading occurs at the lowest possible power, without exporting any power to the grid.
The present invention is oriented towards these needs.